Behaviour
Thecosomata beat their wing-like parapodia to "fly" through the water. They are holoplanktonic; that is, they spend their whole life in a planktonic form, rather than just being planktonic during the larval stage, as is more commonly the case in many marine gastropods, whose veliger larvae are part of the meroplankton.
Little is known about the behaviour of sea butterflies, but they are known to have a peculiar way of feeding. At times, they just float along, ventral-side up, with the currents. They are mostly passive plankton feeders, but at times they can be active feeders. They are generally herbivorous, entangling planktonic food through a mucous web that can be up to 5 cm wide, many times larger than themselves. If disturbed, they abandon the net and flap slowly away. When descending to deeper water, they hold their wings up. Sometimes, they swarm in large numbers and can be found washed up in flotsam, especially along the coast of eastern Australia.
Read more about this topic: Sea Butterfly
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